R=120;
C=33e-12;
tau=R*C;
T=tau/100;
t=0:T:(3*tau);
N=length(t);
U(1)=0;
E=1;
for k=2:N;
    U(k)=U(k-1)-(T/tau)*U(k-1)+E*T/tau;
end
plot(t,U);